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From: pozz <pozzugno@gmail.com>
Newsgroups: comp.arch.embedded
Subject: Re: Voice compression
Date: Mon, 7 Apr 2025 13:13:41 +0200
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Il 05/04/2025 11:12, Rafael Deliano ha scritto:
>> very poor, quite unusable.
> 
> You are not seriously expecting me to debug your code ?

I didn't write this.


> CVSD 16kBit was used in the 70ies for military secure communication.
> The then SpaceShuttle ADM ( = CVSD ) is a simple digital implementation, 
> 16kBit i guess.
> Therefore at 16kBit CVSD is usable, but not for public phone system.
> Initial circuits were analog:
> 
> https://get.hidrive.com/5gdAmSyB   cvsd-ptarmigan.pdf
> 
> The CML FX209 is an early integrated analog version:
> 
> https://get.hidrive.com/HhS2FWU4   cvsd-steele.pdf
> 
> The Harris HC55564 is a simple digital IC.
> 
> The CML FX609 is the next and final generation with PCM-like
> filter that reduces high frequency noise.
> 
> We did use the Harris. On switching to the FX609 had a test with
> all the employees in the company with handset what they liked
> better: 90:10 for the FX609. The problem with "better" is that
> everyone is accustomed to PCM-filtered speech.
> 
> All these ICs one can get via ebay.com from China.
> Using 2 on breadboards one can build a simple channel that
> "distorts" speech for testing. Reference would be an old
> PCM-chip.
> 
> As for quality: the 64kBit PCM may be the gold standard,
> but the cordless DECT phones use ADPCM at 32kBit
> with hardly any loss of quality. This is not the
> original CCITT-ADPCM that was very complex. But i still
> doubt implementation on an AVR is easy.
> 
> CVSDs i did years/decades ago on PICs/68HC05.

 From your last post, it wasn't clear to me if CVSD was a real suggested 
solution for my application.  I tried but the quality is very bad (if my 
code is ok).  Isn't a solution for me for the audio quality we are used 
to these days.

It's much better to reduce the sampling frequency from 8kHz to 6kHz 
without touching anything else in the ADPCM playback system.